This invention relates to cigarette filter rods and cigarettes incorporating such filter rods.
It is a well known fact that the machine or humansmoked "tar" yield of the last few puffs of a conventional filter-tipped cigarette is much greater in quantity than that of the first few puffs Consequently, the "tar" in the first puffs may be perceived by the smoker to be relatively low but in the last puffs may be high enough to be perceived as "too strong".
It has long been an objective of the tobacco industries to produce a cigarette with a more even tar delivery profile. Prior art methods of evening the tar delivery profile have met with varying degrees of success. Kandel, U.S. Pat. No. 3,428,050 proposed to employ a manually adjustable filter. This filter is divided into two parts. By rotating one part of the filter, varying degrees of filtration efficiency can be achieved due to the varying degrees of registration of the high and low efficiency of the filter segments inside the two parts. The disadvantages of this design is that the construction is too complicated for modern high speed filter making machines, and the smoker has to adjust the filter constantly in order to achieve the even tar delivery profile. Browne, et al (Celanese Corp.), U.S. Pat. No. 4,460,001 and U.S. Pat. No. 4,469,112 suggested the use of a compound filter that contains a perforated barrier disc form through which the whole of the smoke passes to a cellulose acetate filter segment. The filtration efficiency of this filter increases as the smoking process proceeds. However, the pressure drop also correspondingly increases to an unacceptable level when enough tar is accumulated on the downstream of the passageways to be effective. Furthermore, this filter is very difficult to make in a modern filter making machine. Norman (Ligett & Myers), U.S. Pat. No. 3,860,011 suggested the use of a hollow tube axially along the centre of the whole filter length in order to allow a portion of the unfiltered smoke to enter into smoker's mouth at high speed. Later, in another patent, U.S. Pat. No. 4,109,666, he suggested to use the same hollow tube configuration, but of shorter tube length, not extending as far as the smoker's mouth, to avoid hot smoke reaching the smoker. The disadvantage of this design is that there is no influence on the tar delivery profile over the whole smoking period, i.e. the tar delivery increases as smoking proceeds. Stewart (Philip Morris), E.P. Patent No. 0 077 123 suggested a fusable tube placed axially at the centre of the filter and the tobacco rod. The tube does not extend the whole length of the cigarette rod, but it is about 5-10 mm short at both ends. In later puffs, the tube fuses and blocks. The design is not practical as it is very difficult to manufacture such filter in a modern filter making machine. GB Patent Specification No. 2 077 570 shows a construction in FIG. 15 comprising a tube which has perforations but the size of the perforations is such that they become clogged after the initial puffs so that the smoke all passes through the outer filter material. Similarly, in the construction described in FIG. 7 in U.S. Pat. No. 3,910,288, essentially all of the smoke passes directly through the outer main filter section once the pathway formed by a channel having an orifice becomes plugged due to the accumulation of "tar" in the area of the filter element adjacent to the orifice.